Exoplanets Continuously Within the Habitable Zone

There are 83 exoplanets in the habitable zone (HZ) in the currently known 694 exoplanets, but many exhibit wide elliptical orbits that take them just inside or outside the edges of the HZ during parts of their orbits. Those that stay within the HZ, instead of just crossing it, are more interesting candidates for habitable exoplanets and exomoons. Here we identified those that stay partially or continuously within the HZ. There are 119 that cross the HZ but only 33 that actually stay within the HZ throughout their orbits (Figure 1 and Table 1). This means that only 33, out of the original 83 exoplanets in the HZ, are better candidates for habitable exoplanets and exomoons.

There are many interesting cases that can be considered. For example, 16 Cyg B b is a gas giant exoplanet in the habitable zone (HZD between -1 and +1) but its wide elliptical orbit takes it outside the inner and outer edges of the HZ. Gliese 581 d and HD 85512 b, the only two known potentially habitable exoplanets, are not always within their HZ. This could impact their habitability. We will be presenting other interesting numbers, such as the range of equilibrium temperatures and time spent in the HZ, for the Habitable Exoplanets Catalog.

Figure 1. Orbits of 694 exoplanets (some outside the range) in habitable zone units (HZU). In these units the HZ has a fixed size and the crossing orbits of exoplanets are easier to visualize. The HZ (light blue shaded area) correspond to HZD values between -1 and +1 HZU. Orbits of exoplanets outside the outer edge of the HZ are shown in blue and those outside the inner edge as red. Those that just cross the HZ are shown in light green and those continuously within the HZ in dark green.